I have had this problem with two companies. With the first company, a small, family run company, I just politely spoke to my boss and it was resolved quickly. This is what I recommend you do initially: speak to your boss or HR manager.

With the second company, it got much more complicated. I was technically going from being on benefits to being self employed, and I now in March 2017, await funds dating back to September 2016. Naturally when I was owed two months payments I resigned. I then had to pursue this through court, and I am still pushing through this. I am also suing for loss of earnings, benefits that I was unable to receive, and further compensation. They are incredibly unhappy about this and have even gone as far as to threaten me, to a point where, except for in court, they are subject to a police restraining order, preventing them from contacting me at all, or being within 100m of me or my home. This sadly is the worst case scenario.

I recommend if they do not permanently resolve the issue and it happens again, you should seek advice from a Citizens Advice Bureau or solicitor promptly. They will be able to recommend next steps. ACAS is also a valuable resource and can be found on Google.

You must remember, you don't work for free and if they don't pay you promptly and you have to pursue official measures, they are only set to lose further!